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Fair Value Measurements - Significant Unobservable Inputs (Details) $ in Millions | Jun. 30, 2021 USD ($) | Mar. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Jun. 30, 2020 USD ($) | Mar. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
--- | --- | --- | --- | --- | --- | ---
LME Swap [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |  |
Fair value | $ (17.9) | $ (6.0) | $ 2.9 | $ 6.1 | $ 7.8 | $ 10.6
LME Swap [Member] | Level 3 [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |  |
Derivative asset, percentage | 0.0858 |  | 0.1000 |  |  |
FX Swap [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |  |
Fair value | $ (0.4) | (0.2) | $ 0.1 |  |  |
FX Swap [Member] | Level 3 [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |  |
Derivative asset, percentage | 0.0858 |  | 0.1000 |  |  |
Nordpool Power Price Swap [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |  |
Fair value | $ 1.5 | $ 0.0 | $ 0.0 |  |  |
Nordpool Power Price Swap [Member] | Level 3 [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |  |
Derivative asset, percentage | 0.0858 |  | 0 |  |  |

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Fair value of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19279-110258 + Details
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